The present invention relates to a disk drive which has a function of compensating a tilt (an inclination) between a disk which is a recording medium and a head from which a light beam is irradiated.
Recently, optical disks have attracted attention as a solution to meet demands for mediums for storing information more voluminous than conventional textual or audio information. On conventional erasable optical disks, guide grooves for use in control to keep a light beam for recording/reproduction on the track center are formed when the disks are manufactured. Because of the guide grooves, lands and grooves are formed on a disk in spiral or concentric form. By the use of both the lands and grooves as recording tracks (land tracks and groove tracks), recording twice as much information as that recorded by the use of either one as recording tracks can be realized. In addition, there is a method of connecting the lands and grooves so that the land tracks and groove tracks alternate every revolution of a disk to form a single spiral track, thereby improving the data access performance. This method is referred to as the single-spiral land/groove. (SS-L/G) recording format. An example of a disk drive using this method is described in Japanese Patent Application Kokai Publication No. 282669/1997.
According to the conventional disk format, recording tracks are divided in the track direction into sectors, and in the forefront of each sector, sector identification information, such as track number and sector number, is pre-formatted as pits which generate variation in physical shape or in local optical constants. Moreover, the sector format includes a first identification information area in which the sector identification information is disposed being displaced a predetermined distance in the radially outward direction from the center of the recording track, a second identification information area in which the sector identification information is disposed being displaced a predetermined distance in the radially inward direction from the center of the recording track, and a user information area which follows the sector identification information areas and in which user information and the like is recorded on the center of the recording tracks.
Next, a disk drive using an optical disk in which sector information is disposed as explained above will be described. FIG. 16 shows a track layout of the conventional optical disk. FIG. 17 is a block diagram showing the configuration of the disk drive for recording or reproducing information on such a kind of optical disks.
FIG. 16 shows the track layout of the conventional optical disk, the disposition of tracks and recording sectors in a zone, and the configuration of each recording sector. As shown in the drawing, the disk is in the SS-L/G recording format and comprises grooves and lands of identical width. That is, the width of grooves and lands is equal to the track pitch, and is a half of the interval between grooves.
In a recording track which includes an integer number of recording sectors, a sector identification information area (a sector identification signal part) in which sector identification information representing information for PLL pulling in, address information and the like is pre-formatted, is added to the forefront of each sector, and a user information area (an information recording part), in which user data and various management information are recordable, is disposed following the sector identification information part.
Moreover, the sector identification information area comprises two parts, that is, the front and rear parts as seen in the scanning direction, and is made of a first identification information area in which the sector identification information is disposed being displaced a predetermined distance in the radially outward direction from the center of the track and a second identification information area in which the sector identification information is disposed being displaced a predetermined distance in the radially inward direction from the center of the track.
An additional function is a track offset compensation. For optical disks of the sample servo method, a method is known which provides a pair of track offset detecting pits in the position displaced a predetermined distance to the right and left sides of the track center on the recording track so as to detect and compensate the tracking offset amount, as shown for example in the optical disk standard ISO/IFC 9171-1, 2 xe2x80x9c130 mm Optical Disk Cartridge Write Once for Information Interchange,xe2x80x9d1990.
When the light beam passes through the center of the pair of track offset detecting pits, the reproduced signal amplitudes of the pair of detecting pits are identical. If the light beam is offtrack, the reproduced signal amplitude of the pit in one side increases while that of the pit in the other side decreases. By detecting the track offset amount of the light beam, and applying compensation, it is possible to realize a control over the light beam so that it passes through the track center. This principle and effect can be applied to the SS-L/G recording format of the conventional drive.
Suppose that the light beam enters, from a user information area (a user signal area) in a certain groove recording sector, into a sector identification information area (a sector identification signal area) in the next groove recording sector. Since the forefront of the sector identification information area is shifted half the groove width to the outer (or inner) periphery of the disk, a corresponding tracking error signal is produced. After a while, the light beam reaches an identification signal part shifted half the groove width to the inner (or outer) periphery of the disk, and a corresponding tracking error signal is output. If these two error signals are detected in waveforms vertically symmetric with respect to a reference level (that is a tracking error level obtained when scanning the track center), the light beam is scanning the track center. Accordingly, the servo can be controlled to keep on the track center, by a comparison in size between tracking errors detected from the identification signal parts displaced to the inner and outer peripheries. Here, the order of disposition of the first and second identification information area is different depending on whether the track is a land or groove. That is, if in a land track the order of disposition is as the first identification information area and then the second identification information area, in a groove track the order is converse.
In this way, providing an SS-L/G recording disk with identification signals also makes it possible to improve a servo characteristic.
With reference to FIG. 17, the configuration of the conventional disk drive is as follows. In the drawing, reference numeral 10, denotes an optical disk, 11 denotes a semiconductor laser (LD) serving as a light source, 12 denotes a collimate lens, 13 denotes a beam splitter, 14 denotes an objective lens, 15 denotes a photodetector, 16 denotes an actuator, 17 denotes a differential amplifier, 18 denotes a difference signal waveform shaping unit, 19 denotes a reproduced difference signal processor, 20 denotes a polarity controller, 21 denotes a polarity reversing unit, 22 denotes a tracking controller, 23 denotes a summing amplifier, 24 denotes a sum signal waveform shaping unit, 25 denotes a reproduced signal processor, 26 denotes a polarity information reproduction unit, 27 denotes an address reproduction unit, 28 denotes an information reproduction unit, 29 denotes a system controller, 30 denotes a traverse controller, 31 denotes a traverse motor, 32 denotes a recording signal processor, 33 denotes a laser (LD) driver, and 34 denotes an actuator driver. The semiconductor laser 11, the collimate lens 12, the beam splitter 13, the objective lens 14, the photodetector 15, and the actuator 16 in combination constitute an optical head which is attached to a head base.
The operation of the conventional disk drive will be described according to the drawing. A laser beam output from the semiconductor laser 11 is collimated by the collimate lens 12, passes through the beam splitter 13, and then is focused onto the optical disk 10 by the objective lens 14. The laser beam reflected from the optical disk 10 contains recording track information, and passes through the objective lens 14, and then is launched onto the photodetector 15 by the beam splitter 13. The photodetector 15, comprising two light detecting parts split by a line extending in parallel with the track in the far-field formed by the reflected light for obtaining a push-pull signal, and two I-V converting units which are correspondent with the light detecting parts, converts the amounts of light detected by the respective light-detecting parts into electrical signals, and respectively supplies the signals to the differential amplifier 17 and the summing amplifier 23.
The differential amplifier 17 generates the push-pull signal by obtaining the difference between the input signals, and supplies the push-pull signal to the difference signal waveform shaping unit 18 and the polarity reversing unit 21. The difference signal waveform shaping unit 18 slices the analog push-pull signal output from the differential amplifier 17, at an appropriate level in order to convert the push-pull signal into a digital value, and supplies the binarized difference signal to the reproduced difference signal processor 19. The reproduced difference signal processor 19 determines the tracking polarity by extracting an identification signal from the binarized difference signal, and supplies a polarity detection signal to the polarity controller 20, the polarity information reproduction unit 26, the address reproduction unit 27, and the information reproduction unit 28.
Receiving the polarity detection signal from the reproduced difference signal processor 19 and a control signal from the system controller 29, the polarity controller 20 supplies a polarity setting signal and control hold signal to the polarity reversing unit 21 and tracking controller 22. The polarity reversing unit 21 judges, based on the control signal from the polarity controller 20, whether the accessed track is in a land or groove. For example, only if the track is judged as a land, the polarity of the output signal of the differential amplifier 17 is reversed, and the output signal is then supplied as a tracking error signal to the tracking controller 22. According to the level of the tracking error signal input by the polarity reversing unit 21, the tracking controller 22 supplies a tracking control signal to the actuator driver 34; the actuator driver 34 provides, according to the signal, a drive current to the actuator 16, and performs a position control over the objective lens 14 in the direction transverse to the recording track. Consequently, the light spot correctly scans the tracks.
At the summing amplifier 23, the output signals from the photodetector 15 are added, and the sum signal is supplied to the sum signal waveform shaping unit 24. The sum signal waveform shaping unit 24 performs, at a given threshold, data slice of a data signal and address signal in analog form so as to make the signals have pulse waveforms, and supplies them to the reproduced signal processor 25. The reproduced signal processor 25 reproduces an identification signal which contains address information and polarity information, from the binarized sum signal obtained by the waveform processing of the sum signal. The polarity information reproduction unit 26 extracts, from the identification signal, the polarity information which indicates the tracking polarity of the sector. The address reproduction unit 27 reproduces sector address information from the identification information. At the information reproduction unit 28, the binarized sum signal representing the user information recorded in the user information area on the disk is decoded and error-corrected, and is output as a reproduced information signal. In the information reproduction unit 28, an analysis of error correction information (for instance, the number of corrected errors and the like) obtained by the error correction or of a jitter enables determination of an data error rate. Generally, the system controller 29 determines the data error rate by reading the error correction information stored in the information reproduction unit 28 as necessary, and through calculation or using a lookup table.
The polarity information output from the polarity information reproduction unit 26 and the sector address information output from the address reproduction unit 27 are sent to the system controller 29, and are used for control over the tracking polarity or sample and hold operation in tracking control. In the configuration under consideration, in order to intercept unwanted disturbances to the tracking servo system, the tracking error signal may be sampled and held immediately before the sector identification information areas, and the tracking control may be kept off while the light beam traces the sector identification information areas. The system controller 29 to which information relating to the identification signals is input from the reproduced difference signal processor 19, polarity information reproduction unit 26, and address reproduction unit 27, supplies control signals to the polarity controller 20, traverse controller 30, LD driver 33, and recording signal processor 32.
The system controller 29 judges whether the light beam is on a desired address, on the basis of information about the identification signals containing address from the address reproduction unit 27 and the like. The traverse controller 30 moves the optical head to a target track, by supplying a drive current to the traverse motor 31, according to the control signal from the system controller 29. At the same time, the tracking controller 22 interrupts the tracking control operation by the control signal from the system controller 29. In normal reproduction, the system controller 29 drives the traverse motor 31 via the traverse controller 30 according to the tracking error signal input from the tracking controller 22, and gradually moves the optical head in the radial direction along with the reproduction.
At the recording signal processor 32, the error correction code is added to the recording data input in recording, and the coded recording signal obtained by the addition of the error correction code is supplied to the LD driver 33. When the system controller 29 sets the LD driver 33 to the recording mode by using the control signal, the LD driver 33 modulates the drive current applied to the semiconductor laser 11 according to the recording signal. The intensity of the light spot irradiated onto the optical disk 10 is thereby varied according to the recording signal, and recording pits are thereby formed. On the one hand, in reproduction, the LD driver 33 is set to the reproduction mode by the control signal from the system controller 29, and controls the drive current so that the semiconductor laser 11 emits light at a constant intensity. In this way, it is possible to detect recording pits and pre-pits on the recording tracks.
In the conventional disk drive in the configuration described above, tracking errors on an optical disk with guide grooves, such as an SS-L/G recording disk, are usually detected by the push-pull method. It is known however that this method in principle is associated with a detected tracking error signal having a waveform vertically asymmetric with respect to a reference level (hereinafter referred to as an optical offset) because of a tilt, even if the light beam is scanning the track center. That is, the effect of the tilt is equivalent to superimposition of an electrical offset on the tracking error signal.
If the optical offset is processed by being regarded as the same as the electrical offset, that is, if an offset which makes the optical offset zero is electrically superimposed for compensation, the light beam runs off the track center (a detrack state), and the reliability in the signal detection is lowered. Specifically, the quality of the signal is degraded (lower S/N), because of a crosserase between adjacent tracks in recording, a poor erasing in overwrite, a crosstalk from an adjacent track in reproduction, and the like. Moreover, the quality of a reproduced signal is deteriorated by an optical aberration caused by the tilt. These problems might be an obstacle to reduction in track pitches as a means for improving the recording density.
To solve the problems, a conventional disk drive uses a method in which the detrack is compensated by utilizing the disk format in which the sector identification information is disposed in a staggered manner in the outer and inner directions with respect to the track center by a certain distance. By the method, the detrack is compensated so that the absolute value of the difference between the tracking error signal and reference level (the level of the tracking error signal obtained when scanning the track center) when the first identification information is being reproduced, and the absolute value of the difference between the tracking error signal and reference level when the second identification information is being reproduced are the same. The method is effective when the detrack is generated by only one cause.
However, in drives practically used, there are other causes of vertically asymmetric tracking error signals, for example, offsets in electrical circuits (hereinafter referred to as electrical offsets for differentiating from the optical offsets), lack of gain balance of an optical system or electrical circuits, and the like. In a broad interpretation, these are regarded as offsets superimposed on tracking error signals.
On this point, the conventional disk drive has a problem that a tilt and detrack cannot be optimally compensated based only on a reflected light from a disk. For this reason, as a method for compensating tilts, an optical tilt detector is additionally provided onto the optical head, and is utilized. By this method, while only tilts can be separately compensated, the optical tilt detector is additionally necessary. Therefore, a problem of inevitable increase in the cost of the drive comes about.
As another method for compensating tilts without the optical tilt detector, the use of a combination of a conventional detrack index and other indexes, such as a jitter and reproduction error rate of a reproduced signal and the like, can be conceived.
In this method, however, there is a problem that tilt and detrack compensation cannot be converged or takes a long time for the convergence, because a tilt compensation through repetitive estimation using a plurality of parameters is needed because the jitter and error rate vary depending upon both the tilt and detrack.
Moreover, methods which adopt a new signal processing method have been studied. In other words, methods by which the detected error rate is lower than in the earlier method have been studied. One of them is the maximum likelihood estimation, for instance, the Viterbi detection known as a detection method which is more defensive against deterioration in the quality of signals than a conventional detection performed bit by bit. This method, however, needs addition of a new configuration to the conventional signal detection system, and increase in the cost of the drive is thereby inevitable.
The invention seeks to solve the problems, and has an object of obtaining a disk drive which can compensate tilts without increase in the cost of the drive and can separately compensate only tilts without influence from the detrack compensation.
According to the invention, the disk drive for compensating the tilt between a disk including a first identification information area shifted radially outward by a predetermined distance from the center of a track, and a second identification information area shifted radially inward by the same distance, and a head for forming a light spot on the disk, comprises a photodetector for detecting light reflected at the light spot, means for determining a sum signal of an output signal from the photodetector obtained when the light spot traces the first identification information area and an output signal obtained when the light spot traces the second identification information area, and tilt control means for using the sum signal as an index to control the relative tilt between the disk and the head so as to cause the index to approach an extreme value.
According to the invention, the disk drive uses a sum signal of a sum signal amplitude from the photodetector obtained when the light spot traces the first identification information area, and a sum signal amplitude obtained when the light spot traces the second identification information area, as the index.
According to the invention, the disk drive uses a sum signal of a difference signal amplitude from the photodetector obtained when the light spot traces the first identification information area, and a difference signal amplitude obtained when the light spot traces the second identification information area, as the index.
According to the invention, the disk drive uses a sum signal of an absolute value of a difference between an envelope of a difference signal from the photodetector obtained when the light spot traces the first identification information area and a reference level, and an absolute value of a difference between an envelope of a difference signal obtained when the light spot traces the second identification information area and a reference level, as the index.
According to the invention, the disk drive for compensating the tilt between a disk including a first identification information area shifted radially outward by a predetermined distance from the center of a track, and a second identification information area shifted radially inward by the same distance, and a head for forming a light spot on the disk, comprises a photodetector for detecting light reflected at the light spot, means for determining a difference signal or an absolute value of the difference signal of an output signal from the photodetector obtained when the light spot traces the first identification information area and an output signal obtained when the light spot traces the second identification information area, and tilt control means for using the difference signal or an absolute value of the difference signal as an index to control the relative tilt between the disk and the head so as to cause the index to approach an extreme value.
According to the invention, the disk drive uses a difference signal or an absolute value of the difference signal of an envelope of the difference signal from the photodetector obtained when the light spot traces the first identification information area, and an envelope of a difference signal obtained when the light spot traces the second identification information area, as the index.